1. Field of the Invention
This invention relates to semiconductor memory devices and more particularly to metal oxide silicon read-only memories of the electrically programmable type.
This invention relates to U.S. patent application Ser. No. 365,828 filed on Apr. 5, 1982.
2. Description of Prior Art
Floating type electrically programmable read-only memories or EPROMS are usually manufactured using cell layouts as seen in U.S. Pat. Nos. 4,112,509, and 4,112,544 issued to Wall and McElroy, assigned to Texas Instruments or in U.S. Pat. No. 3,984,822. Several manufacturers produced EPROM devices of layouts such as this in 8K, 16K, 32K and more recently 64K bit sizes. One of the classic techniques for increasing the array density and read-only type memories is the use of a virtual ground configuration instead of providing a ground line for each column or output line. Virtual ground memories are disclosed in U.S. Pat. No. 3,934,233 issued to Fisher and Rogers or U.S. Pat. No. 4,021,781, issued to E. R. Caudel, both assigned to Texas Instruments. A virtual ground ERPROM layout is shown in U.S. Pat. No. 4,151,021, issued to David J. McElroy, assigned to Texas Instruments. When addressed, the virtual ground EPROM output is in the milivolt range. Because of the small amount of output voltage, the sensing circuit must be extremely sensitive to the small changes in output voltage. As shown by the stated prior art, the sensing circuitry usually includes multiple stage differential amplifier in order to sense the small change in output voltage.
It is an object of this invention to provide an improved electrically programmable read-only memory device that has an output voltage of approximately 1 volt. It is a further object of this invention to provide electrically programmable memory device that employs a simplified sensing circuit.